Since the house was sold in 2021, with a 1099-S issued to the estate, the estate would file a 1041 for the 2021 tax year (either calendar or fiscal).
See https://www.irs.gov/instructions/i1041#en_US_2021_publink1000285977
If this will be the initial and final return, and the sale resulted in a loss, then you can pass that loss through to the beneficiaries on their K-1s.
If you have no experience with pass-through entities or filing for one, you might want to seek professional guidance (although this should be a relatively simple return).
